1. Field of the Invention
The present invention relates generally to unreliable computer networks and more particularly, to transmitted data that is lost, delayed or out of range.
2. Description of the Related Art
Wireless sensors are often deployed in refineries and other manufacturing environments for monitoring applications. These industries may employ data from such wireless sensors and closed-loop process control to maximize performance of reactors, valves and heaters. Industrial process control applications may also benefit from using wireless sensors in this manner. However, closing the control loop over wireless networks is difficult because closed-loop control requires a continuous flow of feedback data from the wireless sensors to a controller. Since wireless networks are frequently subject to interference (i.e., cannot guarantee a timely flow of data), disruption of feedback data may affect a process control application's behavior and performance.
For example, FIG. 1 shows an exemplary environment using wireless sensors associated with a monitoring application where there is a need for closed-loop process control. As illustrated, a monitoring application 101 may receive data via a wireless sensor 102 where the data is associated with a system under control 104. The system under control 104 may be controlled by a controller device 103 but lacks a closed loop to send data from the monitoring application 101 to the controller 103.
FIGS. 2A, 2B, and 2C illustrate another conventional problem by way of an example. In the example, a change in a liquid flow rate through a process heater (e.g., a heater in a refinery) may affect a liquid's temperature and therefore may affect a process' yield. FIG. 2A illustrates a system including such a heater and comprising a closed control loop. As illustrated, a wireless sensor 102 may transmit data comprising a flow rate or other characteristic of a system under control 104 (e.g., a process heater). According to desired operation, the data reaches a controller 203, and the controller 203 responds to the incoming data by sending a control signal 204 to the system under control 202. However, in some scenarios, the data from the sensor 201 may fail to reach the controller 203 or be otherwise corrupted due to wireless interference. In this instance, the controller 203 may send an incorrect control signal 205 to the system under control 202. For example, if the data transmitted by the sensor 201 indicated that a valve of the system under control 202 should be closed, the controller 203 may believe, due to a lack of received data, that the valve should remain open.
In another example, a desired control signal from the controller 203 would cause a flow rate to increase to a steady state level (as illustrated in FIG. 2B). However, interference between the wireless sensor 201 and the controller 203, as illustrated in FIG. 2A, may lead to corrupted sensor data. The corrupted sensor data may provide incorrect feedback to the controller 203, which results in a control signal from the controller 203 that causes the flow rate to continually increase (as shown in FIG. 2C). If the flow rate of the system under control 202 were to continually increase, problems may occur in the resulting process that may adversely affect the control performance and process behavior, and, in some instances, may create undesirable or harmful catastrophes in an industrial environment.
As discussed above, closed-loop process control over wireless networks in industrial environments is limited by the difficulty in obtaining a continuous flow of reliable sensor data from a wireless sensor to a controller. Currently, wireless sensor products have no intelligence, and focus solely on secure connectivity and reporting functions (e.g., alerting and asset tracking). Existing wireless sensor data aggregation products (e.g., wireless gateways and wireless managers) also exhibit no intelligence and typically only relay data they receive from wireless sensors. These products therefore cannot guarantee a continuous flow of wireless sensor data if network interference should occur. Therefore, it appears that no practical systems or methods exist for dealing with data transmission errors that may hinder closed-loop control over unreliable networks such as wireless networks.